Latest Forecast for Mauna Kea Observatories
10 AM HST (2000 UTC) Tuesday 21 February 2017

Warnings
Lingering moisture during the first part of the night

Cloud Cover and Precipitation Forecast
The summit will be mostly clear and stable throughout the night. Nevertheless episodes of high relative humidity are possible during the evening and first part of the night as moisture might linger at the summit then.

Summary of Key Meteorological Variables
Summit temperature will be near -2C this afternoon, -5C this evening and -7.5 tonight. Winds will be from the SSE at 5 to 10 mph. Precipitable water will range between 3 and 4 mm. Observing conditions will be average to less than average (0.6arcsec) and variable in time.

Discussion
The tradewind inversion is expected to cap low-level moisture at or below 7 thousand feet and ensure the summit remains mostly dry and stable at least through Thursday/Friday. Nevertheless patches of moisture and instability aloft might result in afternoon thunderstorms on the slope of Mauna Kea to briefly reach the summit later this afternoon/evening. Snow is possible then and moisture migth linger in the form of fog/ice afterward. The inversion is expected to weaken and possibly lift by the coming weekend and high relative humidity and fog might become again an issue.

Summit skies will be predominantly clear through Thursday night. Some high clouds might be visible along the northern horizon tonight. High clouds will start approaching the area on Friday from the NW and will move closer and over by Saturday when mid level clouds will also join from the SW. As a result the summit sky will be mostly cloudy during the coming weekend.

Precipitable water will range between 3 and 4 mm tonight. It will drop and stay close to 1 mm on Wednesday and Thrusday night and will start raising again afterward to be close to 4 mm on Friday night and likely exceed it on Saturday night.

Observing conditions will be on an somewhat improving trend from tonight through Thursday night. Calm wind at the surface and mostly stable atmosphere should result in average seeing through then but variability will still be an issue though because of shear aloft associated with a strong westerly jet over the islands. Observing conditions will degrade toward the end of the week as result of degrading weather conditions.

The current weather scenario sees a series of high pressure systems passing North of the State, which result on average, on mostly stable conditions. Nevertheless a passing upper level low is advecting a pool of very cold temperature aloft and some mid level instability which might results in afternoon thunderstorms on the slope of Mauna Kea and therefore increase the chances for brief snowvy episodes. This upper level low is expected to move eastward by Wednesday. Another upper level low is expected to form and linger North of the State from the end of this week and through the weekend. This should results in high clouds pushed over the area from the North West and in mid level clouds and moisture advected over from the SouthWest. Instability will increase and so will the chances for more wintery weather at the summit then.